Monolithic implementations of many desirable and important circuits have been hitherto unrealizable, or at least commercially impractical, due to the difficulty in fabricating low loss, expensive, linear passive RF components using conventional fabrication methods. That problem is being addressed, with some success, using micro-electromechanical systems (MEMS) technology. Using MEMS technology, devices having the functionality of inductors, variable inductors and variable capacitors can be realized by various silicon IC-compatible, micron-sized electromechanical structures. The latter component, variable capacitors, are important elements of a variety of electrical circuits including variable-frequency oscillators, tuned amplifiers, parametric amplifiers, phase shifters, equalizers, and impedance-matching circuits, to name just a few.
Variable capacitors are devices in which a change in a control voltage charge or current causes a change in capacitance. One well known implementation of the variable capacitor is the varactor, typically realized as a p-n junction diode. In such a varactor diode, changes in the control voltage can yield up to about a factor of 10 change in capacitance. Diode varactors typically have two ports; an input port and an output port. As a two-port device, diode varactors have limited functionality. In particular, the two signals that are fed to the varactor--a DC bias and an AC signal--are received at the input port. The DC bias sets the capacitance of the varactor diode, while the AC signal is the signal being processed in the circuit that includes the varactor. If both signals are AC, mixing non-linearity disadvantageously occurs such that the response of the varactor to the control signal is non-linear. Moreover, such a two port arrangement disadvantageously introduces DC into the AC signal path.
The aforementioned limitation (i.e., only two-ports) of diode-based varactors has been carried over to most MEMS-based variable capacitors that have been proposed to date. FIG. 1 depicts a simplified schematic of a first MEMS-based variable capacitor 102 in the prior art. Such a variable capacitor typically consists of two parallel plates, 104 and 106. One of the two plates is non-movable. In conventional MEMS-based variable capacitor 102, the non-movable plate is lower plate 106, which is disposed on support or substrate 100. The other of the two plates, upper plate 104 in the present example, is movable. Upper plate 104 is typically suspended over non-movable lower plate 106, such as by beams or suitably arranged hinged plates (not shown).
The two plates are electrically connected to a bias supply (not shown) operable to apply a typically DC bias voltage, V.sub.1, to variable capacitor 102. The two plates are also electrically connected to signal line 110 for supplying a signal, S, typically AC, to variable capacitor 102. As bias V.sub.1 is applied across upper and lower plates 104 and 106, upper plate 104 moves towards fixed lower plate 106. The capacitance of variable capacitor 102 thereby increases. See,Young et al., "A Micromachined Variable Capacitor for Monolithic Low-Noise VCOs," Tech. Digest, pp. 86-89, 1996 Solid State Sensor and Actuator Workshop, Hilton Head Island, S.C., Jun. 3-6, 1996.
FIG. 2 depicts a simplified schematic of a second MEMS-based variable capacitor 202 in the prior art. Variable capacitor 202 has three parallel plates, including non-movable upper plate 206, non-movable lower plate 208 and movable plate 204. Movable plate 204 is sandwiched between the non-movable plates.
The plates are electrically connected to two bias sources (not shown), operable to apply bias voltages V.sub.1 and V.sub.2 to variable capacitor 202 as depicted in FIG. 2. The two plates are also electrically connected to signal line 210 for supplying a signal, typically AC, to variable capacitor 202. When bias V.sub.2 is set to 0 volts and non-zero bias V.sub.1 is applied, movable plate 204 moves upwardly towards non-movable upper plate 206, increasing the capacitance of variable capacitor 202. When bias V.sub.1 is set to 0 volts and non-zero bias V.sub.2 is applied, movable plate 204 moves downwardly towards non-movable lower plate 208, decreasing the capacitance of variable capacitor 202. The three-plate MEMS-based variable capacitor 202 is described, by its inventors, to provide an increased tuning range over a two plate MEMS-based variable capacitor, such as variable capacitor 102. See, A. Dec et al. in "Micromachined Varactor with Wide Tuning Range," Elec. Lett. Online No. 19970628 (Apr. 7, 1997).
In both of the conventional MEMS-based variable capacitors 102 and 202, the bias (V.sub.1 and V.sub.1 /V.sub.2) and signal (110 and 210) are not electrically isolated (i.e., they are applied to the same port). Being two-port devices, MEMS-based variable capacitors 102 and 202 disadvantageously share some of the limitations, such as those described above, common to conventional diode varactors.
The art would thus benefit from a MEMS-based variable capacitor having more than two ports. Such a device would provide a hitherto unachieved degree of flexibility and utility in comparison with conventional diode- or MEMS-based variable capacitors.